A Krueger, or leading edge flap, is a high-lift device deployable from the lower aerodynamic surface of an aerofoil, such as an aircraft wing. When stowed the Krueger trailing edge is disposed at or near the wing leading edge, and a portion of the Krueger device makes up part of the wing lower surface. When deployed the Krueger rotates forwardly from a hinge near the wing leading edge, and the Krueger trailing edge remains adjacent the wing leading edge.
Whilst the Krueger is functionally similar to a slat when deployed, slats are deployed forwardly from the wing leading edge.
Leading edge high-lift devices can be either “slotted” or “un-slotted”. Slotted means that a gap, or slot, is opened up between the deployed high-lift device and the wing leading edge. Un-slotted means that the deployed high-lift device is sealed to the wing leading edge. Airflow through the slotted gap can improve the maximum lift coefficient of the wing in a high-lift configuration, i.e. with the high-lift device deployed.
For best performance the gap between the deployed high-lift device and the wing should be convergent. That is to say, the gap between the wing leading edge and the lower aerodynamic surface of the high-lift device should progressively reduce up to the trailing edge of the deployed high-lift device. If the slot is convergent-divergent the circulation around the high-lift device is reduced due to the less than optimal aerodynamic setting.
A convergent slot can normally be achieved by positioning the deployed device at an appropriate angle and location ahead of the wing leading edge. However, kinematic constraints, particularly for Krueger actuation mechanisms, can limit the positioning of the deployed device and create a convergent-divergent slot. In particular, the available extent of rotation of the Krueger during deployment will affect the angle of the deployed Krueger. Further rotation to an aerodynamically more desirable angle may not be possible without resulting in a more complex, heavier design.